Environmental Engineering Reference
In-Depth Information
fire-fountaining episodes. Witness accounts describe an eruption column more
than9kmhigh(ThordarsonandSelf,
2003
), and the detection of Laki chemicals
in Greenland ice cores con
rms that material reached the stratosphere (Fiacco
et al
.,
1994
;Wei
et al
.,
2008
).
Basaltic eruptions that produce volcaniclastic deposits, however, may have
more capacity to implant material into the stratosphere, particularly at northern
latitudes with a low-altitude tropopause (
9 km near the poles compared to 17 km
at the equator, but sensitive to a wide variety of external parameters (Thuburn
and Craig,
1997
; see also Ross
et al
.,
2008
)). Basaltic volcaniclastic deposits
are commonly phreatomagmatic, and incorporation of water into an eruptive plume
can have a range of effects. If the water vaporizes, the density of the plume
decreases with addition of steam, but the temperature also decreases through
the consumption of latent heat. The decrease of density means a higher plume
may develop with a lower eruption velocity.
Conversely, if added water cannot vaporize, the eruptive fountain will collapse
and
~
flow laterally (Koyaguchi and Woods,
1996
). As phreatomagmatic basaltic
eruptions proceed, they may transition between these states. Koyaguchi and
Woods (
1996
) suggest that accretionary lapilli can form in both the fountains
and wet lateral
find that for an initial eruptive velocity of 100 m/s, a
temperature of 1000 K, and a volatile content of 3 wt% water, a plume may reach
as high as 35 km, at which point its temperature would be below 300 K.
In agreement, Walker
et al
.(
1984
) estimate a vent exit velocity of 250
ows, and
350 m/s
for the basaltic plinian Tarawera eruption of 1886, which generated a column
of ash that ascended
-
~
30 km.
While individual eruptions may not always reach these conditions, the
broad thermal perturbation of a flood basalt province may produce its own weather
pattern of thermals in the atmosphere. This concept was first investigated by
Emanuel
et al
.(
1995
), in which they posited that both very large bolide strikes
and large-scale volcanic eruptions could produce exceptionally violent storms
termed
hypercanes
, capable of injecting large amounts of mass into the strato-
sphere. More recent work by Kaminski
et al
.(
2011
) described penetrative convec-
tion above large lava
flows, where broad temperature perturbations drive large
upwellings past the tropopause.
1.5 Summary: potential for climate change
Ma
cant fraction of large igneous province
eruptive volume, including in the Siberian, Emeishan, North Atlantic,
Karoo, Ferrar, and Columbia River
c volcaniclastics make up a signi
flood basalts. The type and distribution of
volcaniclastics can determine the relative impacts of volcanism and tectonism
